Sunn hemp (Crotolaria juncea) is a fast growing tropical legume that can accumulate large amounts of biomass and N in a relatively short period of time during the summer in the southeast US. This study was conducted to evaluate the potential of using this legume as an N source for cotton (Gossypium hirsutum L.). A field study was conducted in 2004-2005 and 2006-2007. Treatments were summer legume [sunn hemp, cowpeas (Vigna unguiculata), and summer fallow] and fertilizer N rate applied to cotton (0, 45, 90, and 135 kg N ha-1). The summer legumes were planted in mid-summer (July) and cotton was planted the following spring (May). Sunn hemp N accumulation at the first freeze in the fall was 204 kg ha-1 in 2004 and 98 kg ha-1 in 2006. Cowpeas accumulated 57 kg ha-1 in 2004 and 87 kg ha-1 in 2006. Soil NO3-N was higher in the surface 90 cm following sunn hemp than either cowpeas or summer fallow during the winter in both years. Shortly after cotton planting in May, however; there was no difference among the three summer legume treatments in soil NO3-N. Nitrogen concentrations of uppermost fully expanded leaves were greater in cotton following the legumes than following summer fallow at seven of nine weekly sampling dates in 2005, but there were no differences in 2007. In 2005, seedcotton yield following cowpeas and summer fallow increased with N rate, but seedcotton yield following sunn hemp was not significantly affected by fertilizer N. Drought conditions in 2007 limited cotton yield, and yield decreased with increasing N rates due to the severe stress conditions. Sunn hemp has the potential for providing substantial biomass and N for cropping systems on the low organic matter soils of the southeast US. Further research is needed on the fate of the N in the residues during the winter for cotton rotations.
